Optical lenses used in existing digital products are usually ordinary optical lens that has a small viewing angle and low imaging quality. Though wide-angle lenses can overcome that drawback, they have a severe problem, i.e., the image of object is distorted under the optical effect of lens. Due to the fact that only spherical lenses was available in the past, to correct the distortional astigmatism (or referred to as barrel distortion, i.e., the closer the imaging is to the edge, the more severely the image tends to expand and curve linearly), severe meniscus-shaped negative lenses or meniscus-shaped positive lenses have to be added in front of the lens to eliminate the astigmatism. An ordinary wide-angle lens requires 8-10 lenses if the viewing angle of lens is 80 degrees and requires 10-12 lenses if the viewing angle is 100 degrees or higher; as a result, the lens will be too long and too heavy. It is apparently that ordinary wide-angle lenses (especially near infrared wide-angle lenses) are complex in structure and usually have a small relative aperture.
As optical plastic materials are improved and aspherical lens manufacturing technique is popularized, a wide variety of small and light lenses appear. Especially, if aspherical lenses are used, only 3-4 lenses are required if the viewing angle of lens is 80 degrees, only 4-5 lenses are required if the viewing angle of lens is 100 degrees, and only 5-6 lenses are required if the viewing angle of lens is 120 degrees or higher. Apparently, optical lenses are evolving towards miniature and light weight.
If the quantity of lenses is not enough in the lens, it is hard to achieve high imaging quality. To ensure imaging quality, a variety of lenses composed of 4 lenses each are provided in many patents, such as Patent No. CN200610138436.2, CN200710111925.3, CN200710201438.6, CN200810305113.7, and CN200910302836.6, etc. All these lenses employ a structure in which the first lens has positive focal power and other lenses may different in design.
However, it is seen that these lenses are still poor in terms of the wide angle effect, and can't overcome the drawback of image distortion. In addition, all these patents haven't taken account of the effect of temperature fluctuations to the lens. Modern optical instruments usually must have stable performance in a wide range of environmental temperature. Therefore, athermalization design is required for the system. The athermalization design of optical system is to keep the imaging quality of the optical system constant within a wide temperature range with appropriate compensation techniques, usually through three approaches: mechanical active approach, mechanical passive approach, and optical passive approach. Lenses that are commonly used in optical instruments are usually treated in the three approaches to achieve athermalization effect. However, these approaches make the lens structure more complex, and may cause increased lens cost and increased lens size, etc., and therefore are not suitable for portable lenses.